For more than half a century, the cathode ray tube (CRT) has been the principal electronic device for displaying visual information. The widespread usage of the CRT may be ascribed to the remarkable quality of the display characteristics in the realms of color, brightness, contrast and resolution. One major feature of the CRT permitting these qualities to be realized is the use of a luminescent phosphor coating on a transparent faceplate.
Conventional CRT's, however, have the disadvantage that they require significant physical depth, i.e., space behind the actual display surface, making them bulky and cumbersome. They are fragile and, due in part to their large vacuum volume, can be dangerous if broken. Furthermore, these devices consume significant amounts of power.
The advent of portable computers has created intense demand for displays which are lightweight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional CRT, there has been significant interest in efforts to provide satisfactory so-called "flat panel displays" or "quasi flat panel displays," having comparable or even superior display characteristics, e.g. , brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing flat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles is possible, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, thin film field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employs a matrix-addressable array of pointed, thin-film, cold field emission cathodes in combination with an anode comprising a phosphor-luminescent screen. Although the phenomenon of field emission was discovered in the 1950's, extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appear to be promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 August 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 July 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780,"Electron Source with Microtip Emissive Cathodes," issued 16 March 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 July 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
It is important in flat panel displays of the field emission cathode type that the electron emitting surface and the opposed display face be maintained insulated frown one another at a relatively small but uniform distance throughout the full extent of the display face. There is a relatively high voltage differential, generally on the order of 300-1,000 volts, between the emitting surface and the display face, and it is vital that electrical breakdown between these two surfaces be prevented. However, the spacing between the two has to be small, typically on the order of 200 .mu.meters (microns), to assure that the desired thinness, high resolution and color purity are achieved. This spacing also has to be uniform for uniform resolution, brightness, to avoid display distortion, etc. Nonuniformity in spacing is much more likely to occur in a field emission cathode, matrix-addressed, flat vacuum-type display than in some other gas-filled display types, since there is typically also a high differential pressure on the opposite sides of the display face. Whereas the exposed side of such face may be at atmospheric pressure, a high vacuum of approximately 10.sup.-7 tort is generally applied between the emitting surface and the display face of the field emission flat panel display structure.
In general, spacer arrangements of the prior an for field emission-type cathode flat panel displays may be divided into two categories: spacer structures which are firmed as an integral pan of either the emitting structure or the anode structure, and those which are separate from both of these structures, and which are placed between the two during final assembly. In the former category, U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued 15 August 1989, to C. A. Spindt et al., describes a spacer approach in which elongated, parallel legs are provided integrally connected with the display face plate interspersed between adjacent rows of pixels. Another approach, disclosed in U.S. Pat. No. 4,091,305, "Gas Panel Space Technology," issued 23 May 1978, to N. M. Poley et al., for a gaseous discharge type of flat panel display, uses a metal to connect spacers, which metal is then coated with a dielectric layer. This approach is not conducive to being used in a field emission type arrangement, because of the high voltage differential necessary between the anode and cathodes of such an arrangement. This high voltage can exceed the breakdown potential of the dielectric and result in the metal of the spacer posts causing a voltage short between the faceplate and the cathode emitting surface.
Another approach in this category, disclosed in U.S. Pat. No. 4,422,731, "Display Unit With Half-Stud, Spacer, Connection Layer and Method of Manufacturing," issued 27 December 1983, to J.-P. Drogeut et al., is to provide interacting spacer parts on the display face and the cathode construction. U.S. Pat. No. 4,451,759, "Flat Viewing Screen With Spacers Between Support Plates and Method of Producing Sane," issued 29 May 1984, to H. Heynisch, shows such an arrangement for a flat panel display in which metal pins on the face register with hollow cylinders projecting from the cathode. Finally, U.S. Pat. No. 5,063,327, "Filed Emission Cathode Based Flat Panel Display Having Polyimide Spacers," issued 5 November 1991, to I. Brodie et al., discloses polyimide spacers or pillars separating the emitting surface an the display face of a flat panel display.
Many of these prior art approaches of the first-mentioned category have registration problems. All of them add a level of complexity to the fabrication of the cathode and/or anode structure, and all suffer from a performance disadvantage of interfering with the uniform flow of electrons between emitters and anode. It is known that electron beam trajectories avoid spacers shaped as elongated legs, or as cylindrical or rectangular pillars, of the types made of metal, plastic or glass, as disclosed in several prior art references. In these cases the beam cannot penetrate the spacers, and the legs or pillars are likely to be noticeable to a viewer of the display, appearing as dark areas on a luminescent screen.
In the latter category of prior art spacer arrangements, those which are separate from both the cathode structure and the anode structure, U.S. Pat. No. 4,183,125, "Method of Making an Insulator-Support for Luminescent Display Panels and the Like," issued 15 January 1980, to R. L. Meyer et al., discloses a spacer comprising a stack of glass filaments, which are mutually bonded to form a unitary cellular latticework.
In another prior art method of this latter category known to the applicants, uniform spacing between a field emission structure and an anode structure is provided by a multiplicity of glass spheres used as spacers between the cathode plate and the anode plate. These glass spheres, illustratively 200 microns in diameter, serve the dual purposes of providing voltage isolation between the plates, and also provide the standoff of the mechanical forces of vacuum on the two plates. The use of glass spheres as spacers provides a distinct advantage over the pillar structures of the prior art of the first-mentioned category cited above. This advantage is the relative invisibility of the glass spheres in the presence of an electron beam. The trajectory of the electron beam will tend to bend around and follow the circular shape of the spheres, minimizing the area of the display screen which is shadowed by the spacer.
However, there are problems associated with the use of glass spheres as spacers related to handling and assembly. During the fabrication processes of the flat panel display, just prior to assembly of the two halves of the display panel, glue is applied to the planar surface of the emission structure in spots. The spheres are added to the glued surface in excess. Some spheres become attached, the others must be removed, and the glue must be cured. This process can be difficult and time consuming. Similar assembly difficulties are presumed for the FIG. 3 embodiment of the Meyer et al. ('125) reference, comprising a single layer array of loose, unattached parallel filaments.
In view of the above, it is clear that there exists a need for an apparatus for maintaining a uniform spacing between the emission surface and the anode of a field emission flat panel display device which takes advantage of the relative invisibility of the glass spheres, but which lends itself to simpler fabrication processes.